MONSOON-INDUCED CHANGES IN COCCOLITHOPHORID PRIMARY PRODUCTIVITY IN THE BAY OF BENGAL

The Bay of Bengal represents and ideal location to test marine productivity changes in response to monsoon-driven fluctuations in riverine nutrient input. The productivity of marine primary producers may represent an important feedback in the climate system. Using the new productivity indicator, the Sr/Ca ratio of coccolith calcite, we tested recent sediment from the Bay of Bengal collected in traps over a one-year period and monsoon cycle (1994-1995). Data from sediment trap site NBBT-N8 (N 17.5 E 89.5; trap depth 684m) shows the flux of total coccolith carbonate more than doubles between the months of April and September, and has a smaller later surge in early November. Preliminary species separation and microscope observation suggest that this change is due mostly to an increase in F. profunda and G. oceanica during the monsoon months. Sr/Ca analysis shows an increase in productivity of all species beginning in March. All species reach an initial productivity peak in mid-April. F. profunda peaks for a second time in mid-July. All species have a final productivity surge between September and December, but F. profunda and G. oceanica peak in late October while Calcidiscus and Helicosphera peak later, in late November/early December. Between December and March, productivity is low for all species. NBBT-N8 data suggests that monsoon-induced nutrient fluxes do increase coccolith productivity during monsoon months. However, the monsoon cycle does not explain the later productivity surge. We will compare more distal traps from locations NBBT-S8 (N 15.5 E 89.2; trap depth 731 m) and SBBT-08 (05.5 N; 86.75E; trap depth 1919m) to see if response at the locations mirrors that of NBBT-N8 or if the coccolith productivity response varies with distance from the river delta. In Quaternary sediments from the Bay of Bengal, variations in clay mineralogy are interpreted to indicate fluctuations in continental weathering intensity during monsoon cycles (Colin et al., 1999). Using relationships from the sediment trap calibration, and data on coccolith assemblages and Sr/Ca changes over the last 200,000 years, we will evaluate whether monsoon-induced increases in continental weathering affected nutrient supply and productivity.